Hillsboro climate

Hillsboro Frost Dates and Growing Season

NOAA 1991-2020 normals for Portland-Hillsboro AP, 3.8 km from Hillsboro, elevation 204 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Portland-Hillsboro AP. A median date has an even chance of a later freeze in spring, or an earlier one in fall.

The last spring freeze typically comes around April 16 and the first fall freeze around October 23, a growing season of about 191 days.

Median at 36°F
last May 5, first October 6
Median at 32°F
last April 16, first October 23
Median at 28°F
last March 13, first November 12

Last freeze in spring

The chance of a later freeze than the date shown. At 10 percent, one spring in ten still freezes after the date; planting after the 10 percent date for 32°F is the cautious choice.

Threshold10%20%30%40%50%60%70%80%90%
36°FMay 27May 20May 14May 9May 5May 2Apr 30Apr 26Apr 21
32°FMay 3Apr 29Apr 25Apr 20Apr 16Apr 11Apr 5Mar 31Mar 25
28°FApr 9Mar 31Mar 24Mar 18Mar 13Mar 7Mar 1Feb 25Feb 15
24°FMar 5Feb 27Feb 23Feb 19Feb 14Feb 6Jan 28Jan 16Dec 31
20°FFeb 20Feb 12Feb 1Jan 24Jan 9Dec 31Dec 22Dec 11Dec 1
16°FFeb 10Jan 31Jan 15Jan 6Dec 31Dec 21Dec 12Dec 6Nov 30

First freeze in fall

The chance of an earlier freeze than the date shown. At 10 percent, one fall in ten freezes before the date.

Threshold10%20%30%40%50%60%70%80%90%
36°FSep 20Sep 27Oct 1Oct 3Oct 6Oct 9Oct 14Oct 20Oct 31
32°FOct 3Oct 9Oct 14Oct 18Oct 23Oct 28Nov 2Nov 8Nov 18
28°FOct 20Oct 27Nov 1Nov 7Nov 12Nov 18Nov 25Dec 1Dec 10
24°FNov 4Nov 13Nov 20Nov 27Dec 1Dec 9Dec 18Dec 28Jan 11
20°FNov 14Nov 27Dec 1Dec 7Dec 14Dec 25Jan 1Jan 14Feb 9
16°FNov 28Dec 2Dec 7Dec 11Dec 20Dec 29Jan 4Jan 15Feb 4

Growing season length

Days between the last spring and first fall freeze at each threshold, with the chance of a season at least that long.

Threshold10%20%30%40%50%60%70%80%90%
36°F181171163158153147142135127
32°F223212204198191185179170161
28°F286270259250243236229220208
24°F344329315307298291280267255
20°F365365365354343333324314303
16°F365365365365365365365348334

How often it gets that cold

The share of years with at least one night at or below each temperature.

36°F or colder
100% of years
32°F or colder
100% of years
28°F or colder
100% of years
24°F or colder
98% of years
20°F or colder
68% of years
16°F or colder
29% of years

Degree days

A degree day is one degree of the daily mean temperature beyond a base, for one day. Heating degree days count cold (the fuel a house burns), cooling degree days heat, growing degree days the warmth a crop needs; each crop has its own base.

Heating degree days, base 65°F
4,830
Cooling degree days, base 65°F
275
Growing degree days, base 40°F
4,772
Growing degree days, base 45°F
3,331
Growing degree days, base 50°F
2,195
Growing degree days, base 55°F
1,324
Growing degree days, base 60°F
686

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January76103
February63702
March583020
April449165
May2669217
June14728332
July36103532
August36101530
September12433359
October3951113
November614017
December78303

What this means

NOAA computes these from the daily lows at the station over 1991-2020, fitting the dates of each year's last and first freeze to a distribution, so a date can be stated at any level of risk. 36°F is a frost risk for tender plants on a clear, calm night, since the ground cools below the air above it; 32°F is a freeze; 28°F is a hard freeze that kills most annuals.

The station is 3.8 km from Hillsboro. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

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Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USW00094261 (Portland-Hillsboro AP). Public domain.